PROVIDENCE, R.I. – A crucial lead from an anonymous tipster, who had a peculiar meeting with a man on a sidewalk near Brown University, has helped police identify the suspect believed to have killed two students at the university. Just two days later, the same suspect is thought to have fatally shot a professor from the Massachusetts Institute of Technology.

This informant, referred to only as “John” in the Providence police affidavit, is being praised by investigators for providing the essential details that linked the Brown University incident and the murder of an MIT professor in his Brookline home earlier this week.

Following the horrifying event where a shooter fired over 40 rounds inside a building at Brown University, the Providence community has been gripped by fear and frustration, as law enforcement struggled to pinpoint the assailant.

However, after six days of relentless investigation, a breakthrough was made. By late Thursday, police disclosed that they had located the suspect, who had died from a self-inflicted gunshot wound.

The turning point in the case was all thanks to the tipster, John.

“He single-handedly cracked the case wide open,” remarked Rhode Island Attorney General Peter Neronha, acknowledging the invaluable information that led authorities to the suspect within nearly 24 hours.

“When you crack it, you crack it,” he said.

According to police, John had several encounters with 48-year-old Claudio Neves Valente before Saturday’s attack. As police posted images of a person of interest — now identified as Neves Valente — John began posting on the social media forum Reddit that he recognized the person and theorized that police should look into “possibly a rental” grey Nissan. Reddit users urged him to tell the FBI, and John said he did. The police affidavit said they learned about the tip on Dec. 16, three days after the shooting and a day after the tip line was created.

Up until that point, the police affidavit says officials had not connected a vehicle to the possible shooter.

That detail led them to get more video of a Nissan Sentra sedan with Florida plates and enabled Providence police officers to tap into a network of more than 70 street cameras operated around the city by surveillance company Flock Safety.

The affidavit says John gave investigators additional critical details: he encountered Neves Valente in the bathroom of the engineering building just hours before the attack, where John noted the suspect’s clothing was “inappropriate and inadequate for the weather.”

John also bumped into Neves Valente outside, mere blocks from the building, where John watched Neves Valente “suddenly” turn around from the Nissan when he saw John. What ensued was then a “game of cat and mouse,” according to John’s testimony — where the two would encounter each other and Neves Valente would run away.

At one point, John says he yelled out “Your car is back there, why are you circling the block?”

“The Suspect responded, ‘I don’t know you from nobody,’ then Suspect repeatedly asked, ’Why are you harassing me?’” according to the affidavit.

John told police he eventually saw Neves Valente approach the Nissan sedan once more and decided to walk away.

“Respectfully, I have said all I have to say on the matter to the right people,” John wrote on Reddit Wednesday night.

As of Thursday, it’s unknown whether John will receive the $50,000 reward the FBI had offered for information about the Brown shooting.

Ted Docks, special agent in charge of the FBI, said it was possible when asked by reporters.

“It would be logical to think that, absolutely, that individual would be entitled to that,” he said.

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Associated Press writer Matt OBrien contributed to this report.